Radiotherapy consists of projecting onto a predetermined region of a patient's body, a radiation beam so as to destroy or eliminate tumors existing therein. Such treatment is usually carried out periodically and repeatedly. At each medical intervention, the radiation source must be positioned with respect to the patient in order to irradiate the selected region with the highest possible accuracy to avoid radiating adjacent tissue on which radiation beams would be harmful.
A fundamental problem with radiotherapy is the need to position the patient in the same position, when obtaining diagnostic images and each of the subsequent times when radiation is applied to the patient's body. It is to that end that Vision RT have developed an image processing system for monitoring and positioning patients described in U.S. Pat. No. 8,135,201, U.S. Pat. No. 7,889,906, U.S. Pat. No. 7,348,974, and pending US patent applications US2012/0006336 and US2009/0187112 all of which are hereby incorporated by reference.
In use, in the Vision RT system images of a patient on a mechanical couch are obtained by a set of stereoscopic cameras which are then processed to generate a 3D wire mesh model of the surface of a patient being monitored. This 3D wire mesh model is compared with a reference surface created during treatment planning. The relative positioning of the model and the reference surface is compared and used to generate instructions for the mechanical couch to be moved vertically, laterally and rotationally so as to match the surfaces and hence locate the patient reliably in the same location relative to the iso-center of a treatment apparatus. Subsequently during treatment, the position of a patient is continually monitored and if for any reason the patient moves or repositions themselves, this can be detected and if necessary treatment can be halted and the patient repositioned.
Particularly, where the radiotherapy is used to treat cancers inside the cranium, the destruction and damage of nearby healthy tissue can result in severe side effects due to the presence of many critical structures and organs in that part of the body. For this reason, it is very important that radiation is targeted as far as possible so as to destroy only the cancerous cells whilst leaving other nearby structures intact.
Devices immobilizing the position of a patient's head and neck are used to improve the accuracy and reproducibility of positioning the head and neck in medical diagnostic and treatment procedures. Examples of these are described in PCT patent applications WO00/27331, WO03/061477 and WO04/032781 and Vision RT's own pending US patent application US2012/006336. Typically such immobilization devices comprise a cushion and face mask made out of a thermoplastic material which is heated prior to an initial treatment session. The patient lies with their head on the cushion a mask is then molded to a patient's head by being placed over the patient's face and then allowed to set. The resultant rigid mask encloses a patient's head and thus restricts movement and allows a patient to be placed into a fixed position for each treatment session.
Given the importance of accurate positioning for radiotherapy and particularly radiotherapy to treat cancers of the head and neck, it is highly desirable to position a patient with ever greater accuracy. Typically, a mechanical couch is only able to vary the position of a patient, vertically, laterally and rotationally. Although, such adjustments are normally sufficient for most treatments, in the case of cancers of the head and neck, a patient can naturally vary the inclination of their head which adds additional complications to maintaining accurate patient positioning.
Further, it is difficult to make fine adjustments to the positioning of a patient using a mechanical couch. When a patient is lying on a mechanical couch, repositioning a patient requires moving both the patient and the mechanical couch to a new location. This means moving the entire weight of both the patient and the mechanical couch itself. Additionally, most mechanical couches which are able to rotate about an axis of rotation, rotate about an axis which is aligned with a presumed position of the iso-center of a treatment apparatus rather than being able to rotate about the iso-center itself. As a result, almost invariably, some small translations relative to the iso-center are introduced whenever a rotational movement is made which then have to be corrected for separately. This complicates positioning a patient and makes very small (e.g. sub millimeter) patient positioning very challenging and time consuming.
It is therefore desirable to provide a patient positioning system which can help to locate a patient's head in a fixed position to a high level of accuracy and which works with the natural range of movement that is possible for a patient.